NeuroML is a model description language which allows specification of components of biologically detailed neuronal models at many levels (e.g. ion channels, synaptic mechanisms, cells, networks). Examples of NeuroML files specifying each of these are available on the website. There are currently mappings available for most of these elements to the NEURON and GENESIS simulators (and others are being added, e.g. to PSICS.
The latest version of NEURON also natively supports import and export of cells in NeuroML format. neuroConstruct is an application which facilitates the generation of simulation scripts for a number of simulators from models specified in NeuroML. The focus of the language is on multicompartmental conductance based neurons, but some simpler representations of neuronal models (e.g. basic I&F) are supported. The declarative model specifications of NeuroML are complementary to the procedural modelling language approach of PyNN, and work is ongoing to ensure compatibility of these initiatives.
As NeuroML language is being developed as an Open Source project (http://sourceforge.net/projects/neuroml), it is easy to contribute.